The long-standing goal of an effective vaccine against malaria constitutes a crucial component of efforts to eradicate a disease that, according to recent estimates, kills over 1 million persons per year. Malaria vaccines can target sexual and mosquito stage parasite antigens, pre-erythrocytic vaccines that reduce asexual and sexual stage parasite burdens, asexual erythrocytic stage vaccines that reduce blood-stage parasite densities, and vaccines that disrupt parasite development in the vector. So far, vaccines against the early pre-erythrocytic stages have shown most success among current vaccine candidates [1], including the circumsporozoite (CS) protein-based leading vaccine candidate, RTS,S. However, recent results of a phase 3 trial of this subunit vaccine have revealed only modest efficacy of protection against severe malaria [2].
An alternative to subunit vaccine candidates is the use of a whole-organism approach. Such a strategy is based on the generation of immunity by attenuated sporozoites, the Plasmodium form that is injected by an infected mosquito into its vertebrate host. During a natural malaria infection, an asymptomatic parasite maturation and extensive replication phase inside hepatocytes leads to the generation of Plasmodium exoerythrocytic forms (EEFs) and precedes the release of erythrocyte-infectious merozoites that cause disease (reviewed in [3]). A few decades ago, it was shown that sterile protection of humans could be achieved through the injection P. falciparum radiation-attenuated sporozoites (RAS) [4]. More recently, it was shown that sporozoites deficient in certain genes, and which become impaired in complete Plasmodium development inside the liver hepatocyte (GAS), can confer long-lasting protection against malaria in rodents [5]. This has created renewed hopes for a whole-organism vaccine against malaria based on genetically attenuated Plasmodium sporozoites (GAS). Both RAS and GAS are able to invade hepatocytes but fail to complete their developmental process in the liver. Importantly, late liver stage-arresting parasites seem to trigger antimalarial immunity superior to early-arresting variants [6], although they might increase the risk of breakthrough infections.
The protective efficacy of RAS and GAS involves conserved mechanisms and seems to be mainly mediated through the activity of induced CD8+ T cells, although antibodies also contribute to protection. Plasmodium CS is the immunodominant protective antigen in both RAS and GAS [7] and previous studies have shown that protection could be achieved by immunization with CS alone. However, it is also clear that CS is not the sole immunogen at play in the immunity triggered by a whole-organism approach [8, 9].
One major potential drawback of current pre-erythrocytic whole-organism malaria vaccination strategies is that they rely on the attenuation of P. falciparum, the deadliest human-infective parasite species. It has been shown that the radiation dose required to generate effective RAS must be finely tuned to meet minimal requirements. Indeed, sporozoites exposed to high radiation levels will not induce protection, while parasites exposed to low levels will induce breakthrough infection. Similarly to latter, breakthrough infections with different GAS have been reported [10]. Since a single sporozoite undergoing complete development in the liver can give rise to blood infection and malaria symptoms, a vaccination based on the attenuation of P. falciparum sporozoites poses safety concerns that cannot be ignored.
In this context, we hereby propose an alternative strategy for the development of a pre-erythrocytic, whole-organism vaccine against malaria, based on the use of rodent Plasmodium parasites as a non-pathogenic vector of human immunization. Here, we demonstrate that P. berghei is capable of infecting human hepatocytes, as required for optimal antigen presentation, while being unable to cause a blood-stage infection, thereby ensuring the enhanced safety of the proposed approach and we have demonstrated the potential for cross-species protection between rodent and human Plasmodium species.
We further propose that such cross-species protection can be enhanced by the genetic engineering of rodent Plasmodium organisms, to express antigens of their human-infective counterparts. We used a transgenic P. berghei parasite where the endogenous CS has been replaced by that of P. falciparum (PbCSPf) [11] to demonstrate that genetically engineered rodent Plasmodium organisms are able to elicit a specific immune response capable of binding and inhibit infection by P. falciparum. These results compound a new paradigm in malaria vaccination strategies with optimal immunogenic properties.